130 Br J Ophthalmol 2002;86:130 Br J Ophthalmol: first published as 10.1136/bjo.86.2.130 on 1 February 2002. Downloaded from Coverillustration...... My, what big eyes you have...

The Jurassic landscape of 160 million approximately 950 kg, the head being the globe that would have been exter- years ago must have been a fearsome and 20% of that length (Motani R, Sci Am nally exposed, creating an almost com- unforgiving habitat with terrestrial 2000;283:52–9). The young were born plete bony chamber. tetrapods fearing such creatures as Dilo- alive and were probably immediately Perhaps the most prized information phosaurus among other dramatic preda- brought to the surface by the mother or garnered from the fossil record includes tors. The seas of that era contained a instinctively sought the surface as other the eye size and capabilities. The sclerotic group of ferocious predators not yet pelagic birthing marine mammals do plates allow for the calculation of the popularised by movies or toys, or even in today. These air breathing predators diameter of these eyes, and the largest of our common lexicon, but contemporar- evolved from terrestrial ancestors, who these has been found to be 264 mm in ies of the dinosaurs—the ichthyosaurs. themselves had evolved from fish. diameter in a specimen of Temnodontosau- These creatures were highly successful rus platyodon. Ophthalmosaurus had eyes and represent a dynasty of the Jurassic only slightly smaller and some have been found that are 230 mm in diameter, but seas for perhaps 150 million years from this ichthyosaur had a much smaller 245–65 million years ago. Some of the body and weighed far less. larger of these ichthyosaurs could have More information can be determined had Dilophosaurus for lunch if the latter from these fossil records. The f-number were foolhardy enough to enter the shal- of the eye can be approximated with this low seas. information. The f-number is a method Larger and longer than a London dou- of measuring the relative light gathering ble decker bus, Temnodontosaurus or “cut- abilities of the eye—or of a camera lens, ting tooth lizard” successfully ruled the for that matter. The smaller the seas, red in tooth and fin, and was f-number, the more light the system can among the largest aquatic predators ever, absorb and utilise. A human eye has an perhaps with the exception Carcharodon f-number of approximately 2.1, some megalodon, although the two probably Ophthalmosaurus had a long thin man- species of owls approximately 1.1, and never met. dible that would have been an excellent Ophthalmosaurus from 0.8 to 1.1. A large To fill the niche of deeper water, how- tool for capturing fast, manoeuvrable eye such as the one possessed by O iceni- ever, these ichthyosaurs had to develop prey such as large fish and squid, its cus with an f-number of 0.9 would have the visual resources to address these low principal diet. This sleek ichthyosaur was been able to perform well in an extraor- light habitats. The visual imperative of probably active at night, a deep diving dinarily dim environment. An optical feeding at 500–1000 metres or more, species, or both, allowing it to fill a rather instrument that large, with an f-number http://bjo.bmj.com/ instead of the shallow seas of the specialised marine niche. The animal that small, would be an expensive lens continental shelf, would have required possessed a cartilaginous upper tail fin system indeed. the development of large eyes with a which initially puzzled paleontologists Evolutionarily, paleontologists now surprising ability to gather light. because the cartilage was not preserved believe that ichthyosaurs probably Although the ichthyosaurs (and the in the fossil record, and the tail appeared branched off the diapsids (two pairs of much later appearing Carcharodon mega- to bend sharply downward without an openings in the skull in addition to the eyes), at about the time lepidosaurs, lodon) are long extinct, there is much to upper fin. This misunderstanding was on September 24, 2021 by guest. Protected copyright. learn from these spectacular creatures. corrected when an impression of the which includes snakes and lizards, and With eyes larger than footballs, ichthyo- entire fin revealed an upper fin sup- archosaurs, which includes the croco- saurs had the largest eyes ever known, ported by cartilage. The ample fossil diles, birds, and dinosaurs, diverged. and we know much of their lifestyle and record provides much information and These were a distinct group and more habits. allows for understanding of the size, and closely related to reptiles than the Although Temnodontosaurus was the to some extent, the function of the eyes. dinosaurs. The enormous eyes of Oph- second or third largest known ichthyo- The size of the eyes can be determined by thalmosaurus, though, were never re- peated, at least in the vertebrate world, saur but had the largest known eyes ever, the fossilised sclerotic plates or sclerotic although an invertebrate, the giant our cover features an ichthyosaur (Oph- rings that are clearly visible on the cover squid, is still extant and has a similar thalmosaurus icenicus) that was perhaps photograph. These sclerotic plates are sized eye at 250 mm. The giant squid is a more versatile and interesting. O icenicus seen in many other vertebrates, although much larger animal than Ophthalmosau- was another species of parvipelvian ich- not in humans. Birds, especially, have rus, yet probably feeds at similar or even thyosaur with very large eyes, only these sclerotic plates and they serve sev- deeper depths than O icenicus. slightly smaller than the eyes of Temno- eral functions; they help to stabilise dontosaurus,butO icenicus was less than globes that are asymmetric, and probably Ivan R Schwab University of California, Davis, 4860 Y St, Suite half the size, and much more manoeuvra- also helped to support the large eyes in 2400, Sacramento, CA 95817, USA; ble than Temnodontosaurus. Ophthalmosau- these ichthyosaurs against the extreme [email protected] rus, or “eye lizard,” has the distinction of pressure of oceanic depths of 1000 having the largest ocular diameter com- metres of more. As can be seen in at least Thanks to the Natural History Museum (Lon- pared with the body length of any some birds, these sclerotic plates may don) for allowing the photography. ©The Natural History Museum. Thanks to Ryosuke animal, ever. Resembling the morphology even be capable of marrow production. Motani, PhD, for his suggestions and cover of a modern dolphin, O icenicus was only These plates overlap and serve as rein- inspiration. Thanks to Santen, Inc, for the slightly over 4 metres long and weighed forcement for the anterior segment of sponsorship of the cover image.

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